1. Field of the Invention
This invention relates to a roller mill for pulverizing coal, for example, and feeding the pulverized coal to a boiler.
2. Description of the Prior Art
A conventional roller mill will be described with reference to FIGS. 7 through 11. In FIG. 7, reference numeral 13 designates an auxiliary classifier cone whose upper portion is made in a cylindrical shape and whose lower portion is made in an inverted truncated conical shape. In FIGS. 7 through 11, reference numeral 14 designates rotating vanes of a rotary classifier disposed inside the auxiliary classifier cone 13, and which are rotatably supported by a coal feed pipe 20. Reference numeral 15 designates a plurality of deflector plates, 16 a pulverized-coal eject section, and 18 a mill casing. An upper portion and a lower portion of each rotating vane 14 lie in the same vertical plane with respect to the direction of rotation, and the deflector plates 15 are attached to a cylindrical upper portion of the auxiliary classifier cone 13 and are spaced so that individual openings 17 are formed inbetween them. Reference numeral 21 designates a table disposed directly under the coal feed pipe 20, and 22 a roll.
In this roller mill, raw coal is caused to fall through the coal feed pipe 20 onto the table 21 so as to be pulverized by the roll 22, and fine coal particles and coarse coal particles resulting from pulverization are moved upward between the auxiliary classifier cone 13 and the mill casing 18 by means of hot air supplied from below the mill casing 18.
The hot air (upward stream) carrying fine coal particles and coarse coal particles moves across an annular passage formed between an upper portion of the mill casing 18 and the cylindrical upper portion of the auxiliary classifier cone 13, and flows through the openings 17 defined between the deflector plates 15 so as to become a lateral stream flowing toward the rotary classifier. Upon impinging the rotating vanes 14 of the rotary classifier (see FIG. 11), coarse coal particles 6 are sprung back toward the deflector plates 15 (see FIGS. 9 and 10) and are separated from fine coal particles 5; on the other hand, the fine coal particles 5 separated from coarse coal particles 6 are ejected together with the upward stream through the pulverized-coal eject section 16 out of the roller mill. The coarse coal particles 6 sprung back in toward the deflector plates 15 move along the inner surface of the auxiliary classifier cone 13 and fall on the table 21 so that they are repulverized by the roll 22.
The conventional roller mill as shown in FIGS. 7 through 11 has the following drawbacks.
(a) Since the coarse coal particles 6 impinging the rotating vanes 14 are sprung back in FIGS. 9 and 10, they are forced back by the air stream flowing through the openings 17 defined between the deflector plates 15 toward the rotary classifier, so that coarse coal particles 6 are mixed with the air stream carrying fine coal particles 5, resulting in a low efficiency of classification.
(b) Since the deflector plates 15 are attached to the cylindrical upper portion of the auxiliary classifier cone so that the openings 17 are formed between them, the annular passage must be provided around the circular array of openings 17. Thus, the upper portion of the casing 18 corresponding to the cylindrical upper portion of the auxiliary classifier cone 13 must be an enlarged portion, resulting in a correspondingly high cost. Contrarily, if the mill casing 18 were uniform in shape, the auxiliary classifier cone 13 and the rotary classifier 14 will have relatively small diameters and will produce a corresponding increase in the velocity of air flowing through them, resulting in a degradation in classification performance and an increase in pressure loss.